With the rapid development of the wireless communication technology towards the direction of high-capacity, high transmission rate and high reliability, the grave insufficiency of frequency resources has increasingly become a bottleneck which restraints the career development of wireless communication. By summarizing the research achievements of people in wireless communication technology, the most important technology used for improving the spectrum efficiency or increasing the communication capability is the multi-antenna technology.
In the wireless communication, the multi-antennas mainly include three categories. The first category is the sector antenna, it regularly divides the space into a plurality of equal sectors, and signals of all the sectors are non-interfering; The second category is the smart antenna, it can track useful signals in real time and effectively restraint interference signals from other directions at the meantime. The smart antenna technology requires that a spacing between array antennas is about a half of wavelength, so that the signals on all the antennas have better correlated characteristics. The above two categories of multi-antenna technologies mainly use the directivity of array antennas, which belongs to the scope of spatial filtering. The third category is the distributed antenna, which normally uses a reception diversity technique and a transmit diversity technique. The original purpose of using the distributed antennas is to improve the quality of wireless communication in decayed environments. Signals received on all the units can be regarded as independent. In the past, a reception diversity and a transmit diversity are used independently. If the reception diversity and the transmit diversity are used simultaneously, that is, if multiple antennas are simultaneously used to transmit signals at a receiving terminal and a transmitting terminal, such a system is called as a Multi-Input Multi-Output (MIMO) wireless communication system.
Through the perspective of information theory, an MIMO wireless communication system using the distributed antennas has a higher channel capacity than an MIMO wireless communication system using the sector antennas or the smart antenna technology. Meanwhile, with the advance of the Long Term Evolution (LTE) industry, an MIMO antenna system required by the 4th Generation (4G) communication system currently also raises new challenges to the design and evaluation of communication terminal antennas, on one hand, users requires the miniaturized and high-quality user experience, on the other hand, the MIMO antenna system requires that all the antennas have balanced radio frequencies and electromagnetic performances and also have high isolation and low correlation coefficients at the meantime. Therefore, various contradictions have been highlighted in the design of the terminal antennas of LTE system and in the formation stage of system scheme.
At the present, the MIMO technology is used in commercialization in a cell mobile communication system, but applications in the system are also limited by certain factors, and an important limitation factor therein is an antenna. With regard to the antenna array, all factors such as the number of elements thereof, the structure thereof, the placement way of array elements and the form of array elements and so on directly influence the performance of MIMO channels. The MIMO system requires that all antenna elements in the antenna array have less correlation, thus it can be guaranteed that a response matrix of MIMO channels is approximate of a full rank. However, due to the limitations of size and structure of receiver or transmitter of the mobile terminal, antenna elements are always required to be arranged in the extremely limited space as many as possible, and this will make it difficult to implement the high isolation and low correlation of all the antenna elements, which brings great challenges to the design of antenna elements and antenna arrays of the mobile terminal.